Diamond How To Identify Kimberlite

Have you ever imagined holding a rough diamond in your hand, a gem born from the Earth's intense heat and pressure? The journey of a diamond from deep within the Earth's mantle to a sparkling piece of jewelry is a fascinating tale. Central to this journey is understanding kimberlite, the rock formation most commonly associated with diamond deposits.

Imagine a volcanic eruption, not of lava, but of a slurry of rocks, minerals, and gases surging upwards from the Earth’s depths. This is essentially how kimberlite pipes, the primary source of diamonds, are formed. But how do we identify kimberlite and, more importantly, how do we determine if it's the diamond-bearing kind? This article delves into the fascinating world of kimberlite, exploring its formation, identification, and its crucial role in the quest for diamonds.

Main Subheading

Kimberlite is a type of igneous rock that is best known for sometimes containing diamonds. It's named after the town of Kimberley in South Africa, where the first diamond-bearing kimberlite pipe was discovered in the late 19th century. Kimberlite is a rare, volatile-rich (especially carbon dioxide and water), potassic ultramafic rock. This means it's very low in silica and composed mostly of minerals like olivine, serpentine, phlogopite, and garnet.

The formation of kimberlite is a violent process that starts deep within the Earth's mantle, over 150 kilometers (93 miles) below the surface. Here, under immense pressure and temperature, carbon atoms can bond together to form diamonds. Kimberlite magma acts as a carrier, rapidly transporting these diamonds, along with other mantle-derived materials, towards the surface. The eruption is explosive, creating a carrot-shaped structure known as a kimberlite pipe. These pipes are the primary targets for diamond exploration. Kimberlite's significance lies in its ability to bring diamonds from the mantle to the Earth's surface, where they can be mined. Without kimberlite, diamonds would remain inaccessible, hidden deep within the Earth.

Comprehensive Overview

What Exactly is Kimberlite?

Kimberlite is a complex rock with a unique mineral composition and texture. It’s often dark in color, ranging from bluish to greenish to brownish. The rock's texture is typically porphyritic, meaning it contains large, easily visible crystals (phenocrysts) embedded in a finer-grained matrix. This matrix is usually composed of serpentine, calcite, and other alteration products.

The most important minerals found in kimberlite include:

• Olivine: This is a primary mineral in kimberlite, often altered to serpentine.
• Phlogopite: A type of mica that gives kimberlite a characteristic shiny appearance.
• Garnet: Specifically, pyrope garnet, which has a distinctive deep red color and is often associated with diamonds.
• Ilmenite: A black, opaque mineral that can be magnetic.
• Chromite: A dark, metallic mineral containing chromium.
• Diopside: A type of pyroxene mineral.

In addition to these minerals, kimberlite also contains xenoliths, which are fragments of rocks from the mantle and crust that were incorporated into the magma during its ascent. These xenoliths can provide valuable information about the Earth's deep interior. Importantly, while kimberlite is well known for its diamond content, not all kimberlite is diamondiferous, and even within diamond-bearing kimberlites, the concentration of diamonds is typically very low.

The Science Behind Kimberlite Formation

The formation of kimberlite is a complex geological process that is still not fully understood. However, scientists generally agree that it involves the partial melting of the Earth's mantle at depths of 150 to 450 kilometers. The resulting magma is rich in volatile compounds such as carbon dioxide and water. This high volatile content is crucial for the explosive eruption that creates kimberlite pipes. The rapid ascent of the magma prevents the diamonds from converting back to graphite, their more stable form at lower pressures.

The exact trigger for kimberlite eruptions is still debated, but possible mechanisms include:

• Mantle Plumes: Upwelling currents of hot rock from deep within the mantle.
• Lithospheric Extension: Stretching and thinning of the Earth's crust, which can create pathways for magma to reach the surface.
• Faulting: Movement along faults can create fractures that allow magma to ascend.

Kimberlite eruptions are rare events in geological history. Most known kimberlite pipes were formed during specific periods of intense volcanic activity, such as the Cretaceous period (145 to 66 million years ago).

The History of Kimberlite Discovery

The discovery of kimberlite and its association with diamonds revolutionized the diamond industry. Prior to the discovery of kimberlite in Kimberley, South Africa, in the late 19th century, diamonds were primarily found in alluvial deposits, such as riverbeds. These deposits were formed by the erosion of diamond-bearing rocks over millions of years.

The discovery of the Kimberley diamonds marked a turning point. It proved that diamonds could be found in primary rock formations, specifically kimberlite pipes. This led to a surge in diamond exploration around the world, and numerous other kimberlite pipes have been discovered since then.

Some notable kimberlite discoveries include:

• Mir Pipe (Russia): One of the largest diamond mines in the world.
• Ekati Diamond Mine (Canada): The first diamond mine in Canada, which opened in 1998.
• Jwaneng Mine (Botswana): Considered to be the richest diamond mine in the world.

The discovery and exploitation of kimberlite pipes have transformed the diamond industry, making diamonds more accessible and affordable to the public.

Kimberlite vs. Lamproite

While kimberlite is the most well-known diamond-bearing rock, it's not the only one. Lamproite is another type of volcanic rock that can sometimes contain diamonds. Lamproites are also derived from the Earth's mantle, but they have a different mineral composition and form in different geological settings.

Here's a comparison of kimberlite and lamproite:

The most famous example of a diamond-bearing lamproite is the Argyle Diamond Mine in Western Australia, which is known for producing a significant proportion of the world's pink diamonds.

The Role of Indicator Minerals

When exploring for diamonds, geologists often rely on indicator minerals. These are minerals that are commonly found in kimberlite and lamproite, and their presence can suggest the potential for diamond deposits.

Some of the most important indicator minerals include:

• Pyrope Garnet: As mentioned earlier, pyrope garnet is a distinctive deep red garnet that is often associated with diamonds. Its chemical composition can provide clues about the origin of the kimberlite and the potential for diamond content.
• Chromite: This dark, metallic mineral is also a good indicator of kimberlite. The chromium content of chromite can be used to determine the depth of origin of the kimberlite.
• Ilmenite: The presence of ilmenite, especially magnesium-rich ilmenite, can be a positive sign.
• Diopside: Certain types of diopside are indicative of mantle-derived rocks.

Geologists collect samples of stream sediments or soil and analyze them for the presence of these indicator minerals. If indicator minerals are found, they can be traced upstream to their source, which may lead to the discovery of a kimberlite pipe.

Trends and Latest Developments

Today, diamond exploration is more sophisticated than ever before. Modern techniques include:

• Geophysical Surveys: These surveys use various methods, such as magnetic and gravity measurements, to detect anomalies in the Earth's subsurface that may indicate the presence of kimberlite pipes.
• Remote Sensing: Satellite imagery and aerial photography can be used to identify geological features that are associated with kimberlite.
• Geochemical Analysis: Analyzing the chemical composition of rocks and soils can help to identify areas with a high potential for diamond deposits.
• Machine Learning: Sophisticated algorithms are now being used to analyze geological data and predict the location of kimberlite pipes.

One of the most significant trends in diamond exploration is the increasing focus on sustainable mining practices. Diamond companies are under pressure to minimize their environmental impact and to ensure that their operations benefit local communities. This includes reducing water usage, minimizing waste, and creating jobs for local people. There's also growing interest in the ethical sourcing of diamonds, with consumers demanding transparency and accountability in the diamond supply chain.

Another trend is the exploration of deep-sea diamond deposits. While most diamond mining takes place on land, there are also significant diamond deposits located offshore. These deposits are typically found in submerged river channels that were formed during periods of lower sea level. Deep-sea diamond mining is a challenging and expensive undertaking, but it has the potential to become a significant source of diamonds in the future.

Tips and Expert Advice

Identifying kimberlite in the field can be challenging, even for experienced geologists. Here are some tips and expert advice to help you in your search:

1. Look for the Right Geological Setting: Kimberlites are most commonly found in stable continental cratons, which are ancient areas of the Earth's crust that have been relatively undisturbed for billions of years. These areas are typically characterized by flat topography and a lack of recent volcanic activity.
2. Observe the Topography: Kimberlite pipes often form distinctive topographic features, such as circular depressions or raised mounds. These features are caused by the erosion of the kimberlite rock, which is typically softer than the surrounding rocks.
3. Examine the Soil: The soil around kimberlite pipes is often different from the surrounding soil. It may be darker in color, richer in certain minerals, and contain indicator minerals such as pyrope garnet and chromite. Pitting and trenching to expose subsurface soils can be extremely helpful.
4. Analyze the Rock Texture: As mentioned earlier, kimberlite typically has a porphyritic texture, with large crystals embedded in a finer-grained matrix. Look for the presence of olivine, phlogopite, garnet, and other characteristic minerals. A hand lens or magnifying glass can be helpful for identifying these minerals.
5. Use a Magnet: Ilmenite, a common mineral in kimberlite, can be magnetic. Use a magnet to test rock samples for the presence of ilmenite. A strong magnetic attraction can be a positive sign.
6. Collect Samples for Laboratory Analysis: The most reliable way to identify kimberlite is to collect samples and send them to a laboratory for analysis. Geochemical analysis and mineralogical analysis can confirm the presence of kimberlite and provide information about its potential for diamond content.
7. Consult Geological Maps and Data: Before you start exploring, consult geological maps and data for the area you are interested in. These resources can provide valuable information about the geology of the area and the location of known kimberlite occurrences. Government geological surveys are a great place to start.
8. Work with Experienced Geologists: If you are serious about finding kimberlite, it's best to work with experienced geologists who have expertise in diamond exploration. They can provide guidance and advice and help you to avoid common mistakes. Their expertise can be invaluable in interpreting geological data and identifying promising exploration targets.
9. Be Patient and Persistent: Diamond exploration is a challenging and time-consuming process. It can take years of exploration and analysis to find a diamond-bearing kimberlite pipe. Be patient and persistent, and don't give up easily. Remember that even experienced geologists often face setbacks and disappointments in their search for diamonds.

FAQ

Q: What is the difference between primary and secondary diamond deposits?

A: Primary diamond deposits are those found in their original source rock, which is typically kimberlite or lamproite. Secondary diamond deposits are those that have been eroded from their primary source and transported to a new location, such as a riverbed or beach.

Q: How deep are kimberlite pipes?

A: Kimberlite pipes can extend for hundreds of meters or even kilometers below the surface. The depth of a kimberlite pipe depends on the size of the eruption and the geological conditions in the area.

Q: Are all kimberlite pipes diamond-bearing?

A: No, not all kimberlite pipes contain diamonds. In fact, only a small percentage of kimberlite pipes are diamondiferous. Even within diamond-bearing kimberlites, the concentration of diamonds is typically very low.

Q: What is the average grade of a diamond mine?

A: The grade of a diamond mine is measured in carats per tonne (cpht), which is the number of carats of diamonds found in each tonne of rock. The average grade of a diamond mine is typically very low, often less than 1 cpht.

Q: How are diamonds extracted from kimberlite?

A: Diamonds are extracted from kimberlite using a variety of methods, including crushing, screening, and density separation. The goal is to separate the diamonds from the surrounding rock and minerals. The process often involves heavy machinery and sophisticated technology.

Conclusion

Identifying kimberlite is a crucial step in the diamond exploration process. By understanding its formation, composition, and geological setting, you can increase your chances of finding these valuable rocks. Remember to look for the right geological environment, observe the topography, examine the soil, and analyze the rock texture. And, most importantly, be patient and persistent in your search.

Do you have any experiences with rock and mineral identification? Share your stories or questions in the comments below! We'd love to hear from you and continue the conversation about the fascinating world of geology and diamond exploration.